while there is no bar for other lines, wait for bar to synchronize

Author: Karl Vernet

backtrader/feeds/rollover.py

@@ -169,7 +169,8 @@ def _load(self):
             for i, d_dt in enumerate(zip(self._ds, self._dts)):
                 d, dt = d_dt
                 while dt < dt0:
-                    d.next()
+                    if d.next() is None:
+                        continue
                     self._dts[i] = dt = d.datetime.datetime()
 
             # Move expired future as much as needed